The present invention relates to an ink-jet color printing apparatus for forming, that is, painting color images with several kinds of colored inks and more particularly to an apparatus which is suitable for painting color images having half tones such as a color photograph.
Ink-jet printing apparatus in general use is constructed so that ink drops are ejected from an ink-jet head toward a recording paper and then deposited thereon in order to form the desired images. An apparatus wherein characters, pictures or the like are recorded in monochrome with a single-color ink, has the advantage that a high-speed recording can be performed without noisy operation; hence it is widely employed as an output unit of facsimile machines and computers.
Recently, ink-jet color printing apparatus using a plurality of colored inks, namely, yellow, magenta and cyan, has been proposed, wherein a printing head assembly is arranged opposed to a recording paper wrapped around a rotary drum rotating at a fixed speed (the direction of the rotation of the rotary drum is hereinafter called the primary scanning direction) and adapted to move intermittently in a direction perpendicular to the primary scanning direction (which latter direction is hereinafter called the secondary scanning direction) once each revolution of the rotary drum. The printing head assembly has three ink-jet heads for ejecting several colors of ink drops arranged in a row in the primary scanning direction so as to deposit ink drops on a single scanning line (that is, a picture elements line).
In the apparatus described above, however, there is the disadvantage that ink drops will be turbid or caused to flow due to the movement of the recording paper since the color drops are laid on the others previously deposited on the recording paper before the said other ink drops soak in or dry.
In the case of printing color images having half-tones and hues such as color photographs, it is necessary to be able to reproduce picture images with half-tones and hues closely similar to the original in at least sixteen steps of gradations. A drop-on-demand type of ink-jet head, whereby ink dots can be varied in size in accordance with voltages applied, is suitably used in general and is well known in this art. In this type of ink-jet head, ink drops are practically limited from 100 to 180.mu. in size so that images with half-tones in sufficient steps of gradation are hardly obtainable. To avoid the problem described above, it has been proposed to vary the number of ink dots appearing on a dot matrix having n possible positions in the row and m possible position in the column (n and m being integers) for one picture element so as to reproduce images with half-tones in a sufficiently large number of steps of gradation.
In the conventional apparatus described above, there is additionally the problem that in the case of using dot matrices, high speed printings are hardly achievable because of the provision of an ink-jet head for each color. Although the problem encountered could be dealt with by employing a plurality of ink-jet heads, one for each color, the result would be the arrangement of a plurality of ink-jet heads for the same color in the secondary scanning direction since ink-jet heads for a plurality of colored inks are arranged in the primary scanning direction.
In the case of the arrangement in the secondary scanning direction as described above, there are, due to the bulk of the ink-jet heads, spaces equal to several tens of scanning lines between two dot lines to be formed by two ink-jet heads of the same color. Therefore, the ink-jet heads additionally provided must be delayed in their actuation for a relatively long period of time; and hence it is necessary to provide a large scale delay circuitry.